The present invention relates to a display device for displaying by applying a drive signal to a display-use pixel electrode through a switching element, more particularly relates to a matrix-type liquid crystal display device, which permits a high density display by disposing pixel electrodes in a matrix form, and also relates to a method of compensating for a defective pixel electrode in such display device.
A conventional display device such as a liquid crystal display device, a plasma display device, etc., includes a plurality of pixel electrodes disposed in a matrix form, counter electrodes facing these pixel electrodes, and a display medium (liquid crystal, plasma, etc.,) sealed between the pixel electrodes and the counter electrodes. The described display device selectively applies a voltage to the pixel electrodes to form a display pattern on a screen. Further, by applying a voltage between the selected pixel electrode and the counter electrode, the brightness of the display medium is optically modulated by the display data to visualize the display pattern.
For the method of driving the pixel electrodes, a so-called active-matrix driving system is known wherein switching elements are connected to respective pixel electrodes disposed in a matrix form and the pixel electrodes are respectively driven by the switching elements. For the switching element, a TFT (thin-film transistor), and an MIM (metal-insulator-metal) element, etc., are generally known. On the other hand, the pixel electrodes are typically formed on the substrate in the same layer as signal lines, scanning lines (bus lines) in such a manner that they do not contact the signal lines or the scanning lines.
Additionally, the technique of forming pixel electrodes on a different layer from the bus lines by disposing the pixel electrodes on an insulting film is proposed (Japanese Laid-Open Patent Application No. 156025/1986 (Tokukaisho 61-156025). In the described arrangement, as the pixel electrodes and the bus lines are formed on different layers, an increased area of the pixel electrodes (aperture ratio) can be achieved.
The liquid crystal display device adopting the matrix-type substrate always faces a problem of a disconnection of wire due to a defect generated in the manufacturing process. In order to suppress the generation of such disconnection defect, the active-matrix type liquid crystal display device which adopts double bus lines has been proposed (SID ""95 DIGEST of TECHNICAL PAPERS 4: AMLCDs 4.3; xe2x80x9cHigh-Aperture and Fault-Tolerant Pixel Structure for TFT-LCDsxe2x80x9d).
As shown in FIG. 14, the described active-matrix type liquid crystal display device is arranged such that two scanning lines 52 and 52xe2x80x2 are formed for each pixel electrode 5l, and the scanning lines 52 and 52xe2x80x2 are short-circuited by short-circuit lines 54 formed along signal lines 53 on both sides of the pixel electrode 51. The short-circuit lines 54 are superimposed on the pixel electrode 51 via an insulating film (not shown), and an overlapped portion functions as an auxiliary capacitance. In the described arrangement, as a TFT 55 is driven by the two scanning lines 52 and 52xe2x80x2, even if a disconnection occurred in one of the scanning lines 52 and 52xe2x80x2, an application of the gate voltage to the TFT 55 can be ensured through the short-circuit lines 54.
In general, in order to prevent light from leaking through a gap formed between the pixels, a light-shielding pattern is formed on the side of the counter electrodes. In the described arrangement, however, the pixel electrode 51 and the short-circuit lines 54 are superimposed in a direction perpendicular to the substrate. Therefore, the short-circuit lines 54 form a part of the light-shielding pattern.
The arrangement where the pixel electrode and the signal line are superimposed via the insulating film will be explained.
In the arrangement shown in FIG. 15, peripheral portions on both sides of the pixel electrodes 51 are superimposed on the scanning lines 52 and the signal lines 53. As shown also in FIG. 16, at a central portion below the pixel electrode 51, formed is an auxiliary capacitance electrode (hereinafter referred to as Cs electrode) 56. The Cs electrode 56 is formed on a gate-insulating film 57 used in common with the TFT 55 (see FIG. 15). The Cs electrode 56 is in contact with a contact portion 51a of the pixel electrode 51.
On a substrate 58 made of glass, formed is an auxiliary capacitance line 59. The gate insulating film 57 is formed so as to cover the auxiliary capacitance line 59. On both sides of the Cs electrode 56 on the gate insulating substrate 57, lower signal lines 60 are formed, and further, signal lines 53 are formed thereon. The lower signal lines 60 and the signal lines 53 are covered with an insulating substrate 61.
In the described arrangement, as the insulating film 61 is formed between the pixel electrode 51 and the signal lines 53, an increased area of the pixel electrode 51 can be obtained irrespectively of the disposed positions of the signal lines 53.
The arrangement shown in FIG. 17 includes the Cs electrode 56 having the same structure as that of the aforementioned arrangement of FIG. 15, except that the Cs electrode 56 is connected to a drain electrode 62 through a connection line 63. The arrangements shown in FIG. 15 and, FIG. 17 both have the Cs-on-Common structure wherein an auxiliary line capacitance is formed by disposing the Cs electrode 56 on the common auxiliary capacitance line 59 which is used in common among all the pixels.
On the other hand, the arrangement shown in FIG. 18 has the Cs-on-Gate structure wherein an auxiliary capacitance is formed by disposing the Cs electrode 56 on the scanning line 52 of an adjacent pixel. In this arrangement, the Cs electrode 56 is connected to a contact portion 51b of the pixel electrode 51.
In the arrangement shown in FIG. 19, the Cs electrode 56 is connected to the drain electrode 62 through the connection line 63.
With a demand for higher definition and higher aperture ratio, there is a tendency of reducing the width of the bus line while increasing the number of the bus-line crossing parts, which increases a disconnection of a bus-line or a leakage at a portion where the bus-lines are crossed. Furthermore, such disconnection of bus-line, or the leakage at the crossing point causes a problem that a voltage cannot be applied properly to the pixel electrode connected to the disconnected bus line. Therefore, the portion where the voltage is not applied appears as a line-shaped defect on the display screen. In the display element, such line-shaped defect is a serious problem, and a display device having such line-shaped defect is considered as an inferior good. Further, an increase in such inferior goods would lower the yield of the display device, thereby increasing a manufacturing cost.
Furthermore, when the described arrangement of adopting the double bus line is applied to the general arrangement where the pixel electrode and the bus line are formed in the same layer, as the pixel electrode is formed in the same layer as the bus line, an increased area of the pixel electrode cannot be obtained, thereby hindering an improvement of the aperture ratio. Although a small improvement in aperture ratio can be achieved by reducing an interval between the wires; this would causes the problem that a leakage between the wires is likely to occur.
In the arrangements shown in FIG. 15 through FIG. 19, it is permitted to arrange such that the pixel electrode 51 and the data electrodes 53 are superimposed. However, the capacitance between the pixel electrode 51 and the signal line 53 cannot be made smaller due to the insulating film 61 formed therebetween. Therefore, the problem of generating crosstalk due to the capacitance, which would lower the display quality remains unsolved.
An object of the present invention is to provide an active-matrix type liquid crystal display device whose structure permits a generation of a line-shaped defect to be prevented and an improved aperture ratio to be attained with ease.
In order to fulfill the above object, an active-matrix type liquid crystal display device in accordance with the present invention which includes a plurality of scanning lines formed on a substrate; a plurality of signal lines formed so as to cross the scanning lines at right angle; a pixel electrode formed in a region surrounded by adjacent scanning lines and adjacent signal lines; and a switching element for switching ON/OFF an application of a signal voltage to the pixel electrode through the signal line by a scanning voltage to be applied to the scanning line is characterized by further including the following means.
Namely, the first active-matrix type liquid crystal display device includes auxiliary lines formed in the same layer as the signal lines. Each auxiliary line short-circuits two portions of the signal line for applying a signal voltage to corresponding two pixel electrodes which are adjacent to one another along the signal lines.
The second active-matrix type liquid crystal display device is arranged so as to include auxiliary lines formed in the same layer as the scanning lines. Each auxiliary line short-circuits two portions of the scanning line for applying a scanning voltage to corresponding two pixel electrodes which are adjacent to one another along the scanning line.
According to the first active matrix-type liquid crystal display device, two portions of the signal line are short-circuited by the auxiliary line. Therefore, when a disconnection defect occurred in the signal line, a signal voltage would be applied to the signal line through the auxiliary line so as to make a circuit around to avoid the disconnected portion. This feature offers a particular effect that even if a disconnection defect occurred between a certain pixel electrode and a next pixel electrode, the signal voltage can be kept applying to the pixel electrode.
Similarly, in the second active-matrix type liquid crystal display device, when a disconnection defect occurred in the scanning line, a scanning voltage is applied to the scanning line so as to make a circuit around to avoid the disconnected portion. Therefore, the scanning voltage can be kept applying to the pixel electrode.
As a result, a generation of a line-shaped defect is prevented, and good products can be obtained at significantly improved yield. Here, the following problem possibly occurs: That is, a signal line which starts being disconnected is finally disconnected after the active-matrix type liquid crystal display device is delivered to the user. The described active-matrix type liquid crystal displays of the present invention provide the solution to the described problems and permits a display quality to be ensured even in the described situation. Therefore, an active-matrix type liquid crystal display device permits a reduction in manufacturing cost while improving a reliability.
As a preferred form of the first or second active matrix-type liquid crystal display device, it may be arranged such that each auxiliary line is connected to a single signal line or a single scanning line at one portion at a predetermined interval from the crossing point between the signal line and the scanning line per each pixel region.
The conventional arrangement where the auxiliary line is connected to the signal line or the scanning line at a plurality of portions formed at different intervals from the crossing point between the signal line and the scanning line per pixel region has such a drawback that the signal line or the scanning line between the connecting portions will not be short-circuited to the signal line or the scanning line corresponding to the adjacent pixel electrode. In contrast, the described arrangement of the present invention eliminates the described problem, i.e., the defective portion where the signal line or the scanning line is not short-circuited by the auxiliary lines by specifying the connecting portion between the auxiliary line and the signal line or the scanning line.
The auxiliary line of the present invention is arranged so as to have a minimum length required for applying a signal voltage or the scanning voltage in replace of the signal line or the scanning line at an occurrence of disconnection defect. Therefore, even if a material of a large specific resistance, such as ITO, etc., is adopted for the auxiliary line, an overall increase in resistance of the wires can be suppressed, thereby preventing the deterioration of the display characteristics. Therefore, the active-matrix type liquid crystal display device can be achieved with an improved yield at low cost with high reliability.
As another preferred form of the first or the second active-matrix type liquid crystal display device, it may be arranged such that each auxiliary line is formed in a width narrower than that of the signal line or the scanning line to be connected thereto.
The feature that the auxiliary line is formed narrower than the signal line or the scanning line offers an effect that the region where light is blocked by the auxiliary lines in the pixel can be reduced, thereby suppressing a drop in aperture ratio of the pixel. Moreover, an increase in parasitic capacitance between the signal line or the scanning line and the pixel electrode can be suppressed.
As a still another preferred form, the first or second active-matrix type liquid crystal display device may be arranged such that each auxiliary lines are made of a transparent electrically conductive material.
This feature offers an effect that the light that is transmitted through the pixel will not be blocked by the auxiliary lines, thereby preventing a drop in aperture ratio of the pixel. As a result, the active-matrix type liquid crystal display having the described arrangement offers an improved display quality.
As a still another preferred form of the first or second active-matrix type liquid crystal display device, it may be arranged such that the pixel electrodes is formed on an organic insulating film which covers the signal electrode.
In general, as an organic insulating film has a low dielectric constant, a capacitance between the pixel electrode and the signal line can be reduced. Also, the capacitance between the scanning line formed under the signal line and the pixel electrode can be reduced. Therefore, a generation of crosstalk due to the capacitance formed between the pixel electrode and the signal line can be suppressed, and also the pixel voltage that is pulled-in due to the capacitance formed between the scanning line and the pixel electrode can be suppressed. Therefore, the active-matrix type liquid crystal device permits an improved display quality by suppressing the effect from each of the described capacitances.
As a still another preferred form of the first or second active-matrix type liquid crystal display device, it may be arranged such that an insulating film is formed between the pixel electrodes and the signal lines or the scanning lines. On the other hand, two auxiliary lines are respectively connected to two adjacent signal lines or two adjacent scanning lines which are disposed so as to surround the pixel electrode. According to the described arrangement, capacitances are respectively formed between the pixel electrode and one auxiliary line and between the pixel electrode and the other auxiliary line, and the auxiliary lines are formed in such a manner that the described two capacitances are equal. Further, a signal voltage whose polarity reverses per every line is applied to the signal line.
For example, the auxiliary lines are disposed in such a manner that portions connected to one signal line or scanning line are alternately formed between both sides of the signal line or the scanning line. In other words, the auxiliary lines are disposed in such a manner that a plurality of the auxiliary lines are connected to one signal line or scanning line on its opposite sides every one auxiliary line.
In the arrangement where the signal lines are formed in the described pattern, and a signal voltage whose polarity reverses per line is applied to the signal lines, influences from respective capacitances between the pixel electrode and the auxiliary lines can be cancelled out, thereby reducing an influence of the capacitance. As a result, the active-matrix type liquid crystal display having the described arrangement permits a display quality to be improved by reducing a generation of crosstalk due to the capacitance.
The first method of compensating for a defective pixel in accordance with the present invention for the first active-matrix type liquid crystal display device is characterized in that the signal line is cut off on both sides of the scanning line when a leakage defect occurred at a crossing point between the scanning line and the signal line.
According to the described method, when a leakage defect occurred in the crossing point between the scanning line and the signal line, the signal line is disconnected on both sides of the scanning line. After the signal line is disconnected, a voltage is not applied to the signal line at the connecting portion, thereby preventing a generation of leakage. Moreover, even after the signal line is disconnected, as a voltage can be kept applying to the signal line by the auxiliary lines, a generation of a defective pixel can be prevented. As a result, the method of compensating for a defective pixel electrode permits an improved signal quality by eliminating the leakage defect.
The second method of compensating for a defective pixel designed for the second active-matrix type liquid crystal display device is characterized in that the scanning line is cut of f on both sides of the signal line when a leakage defect occurred at a crossing point between the signal line and the scanning line.
The described method offers the same effect as achieved by the first method of compensating for a defective pixel electrode. That is, by cutting off the scanning line, a voltage is not applied to the connecting portion of the scanning line, thereby preventing a generation of leakage. Moreover, even after the signal line is disconnected, as a voltage can be kept applying to the scanning line by the auxiliary lines, a generation of a defective pixel can be prevented. As a result, the method of compensating for a defective pixel electrode permits an improved signal quality by eliminating the leakage defect.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved treatment method, as well as the construction and mode of operation of the improved treatment apparatus, will, however, be best understood upon perusal of the following detailed description of certain specific embodiments when read in conjunction with the accompanying drawings.